U.S. Pat. No. 4,385,551 discloses a laboratory fume hood with a pair of vertically movable sashes and with a locking device that operatively couples the sashes so that one sash must be closed before the other sash may be opened. Such an arrangement allows the use of a relatively large workbench for laboratory experiments and a hood with an ample opening for access to the workbench for setting up and conducting laboratory experiments. Since both sashes cannot be raised at the same time, however, the arrangement prevents excessive amounts of heated or cooled room air from being drawn into the hood while a laboratory experiment is in progress. Perhaps more significantly, the fact that only one sash may be opened at any given time insures that noxious gases will not escape into the room when access to the work area is desired because the more limited area of the opening (in comparison with the total area of the access opening if both sashes could be fully lifted) results in greater air flow velocity through the hood opening.
Other fume hoods have been commercially available with pairs of vertically-movable sashes but, unlike the patented construction, permit both sashes to be partially raised simultaneously. Such an arrangement is advantageous because it permits the entire work area to be exposed at one time. Protection against the system becoming overloaded and resulting in possible backflow into the room is prevented by the latching means that prevents both sashes from being opened more than a limited distance (e.g., no more than halfway) when the hood is in operation.
In some instances the nature of the particular laboratory experiments or the apparatus involved may make it preferable to be able to partially open both sashes at the same time, whereas in other instances it may be more desirable to have the benefits of the patented construction that permit either one of the sashes to be opened fully while the other is locked closed. Regrettably, no prior constructions are known that give the user such a choice.
An important aspect of this invention therefore lies in providing a laboratory fume hood having two operating modes that either permit both of the vertically-movable sashes to be raised into partially-opened positions, or allow one or the other of the sashes to be lifted into a fully-opened position with the other sash automatically locked closed, whichever mode the user selects. The locking mechanism is totally mechanical (in contrast to one depending on electronic components for its bifunctional locking actions) and is highly effective and reliable as well as being relatively uncomplicated and inexpensive in construction. In a preferred form, the locking mechanism is selectively adjustable into a third operating mode that permits both sashes to be fully raised simultaneously when, for example, the hood is to be cleaned or experimental apparatus is to be erected or disassembled, during which time the fume-exhausting operation of the hood is not required. If desired, an alarm system, either audible, or visible, or both, may be incorporated so that the user will be warned against unintentionally overloading the air exhaust system by fully opening both sashes while an experiment is in progress.
Briefly, the multifunctional locking mechanism includes a horizontal shaft that is both slidably and pivotally supported by the fume hood cabinet above the access opening of that cabinet, the shaft having its axis extending along a vertical plane between and perpendicular to the sashes. The longitudinal axis of the shaft is also normal to the common vertical plane of the sashes. The shaft, which may be equipped with a knob to facilitate manual operation, is axially slidable between at least two positions of adjustment. A stop plate constituting first-stop means is mounted on the shaft and is engagable with each of the sashes when they are partially opened, but only when the shaft is in its first position of adjustment, for preventing each of the sashes from being fully opened. A second plate constituting second stop means is also mounted upon the shaft and provides a pair of abutment surfaces facing in different directions and selectively engagable with the sashes when the shaft is in a second position of adjustment. The second stop plate and the shaft on which it is mounted are automatically pivoted one way or the other when either sash is fully raised, the second stop plate when so pivoted serving to block the other sash from being even partially raised. Thus, by sliding the shaft of the multifunctional locking mechanism into either its first or second positions of adjustment, a user may lift both sashes partially or, alternatively, lift only one of the sashes fully.
In the preferred embodiment of the invention, the shaft may also be shifted axially into a third position of adjustment in which neither of the stop plates is engagable with the sashes. Under such circumstances, both of the sashes may be fully raised to expose the work area to its fullest extent, for cleaning, for the setup or disassembly of equipment, or for any other purpose. Optionally, signalling means emitting either a visible or audible signal, or both, may be provided to warn the user that both sashes have been fully opened and that operation of the hood under such conditions could be dangerous.
Other features, objects and advantages of the invention will become apparent from the specification and drawings.